vulkan: do not distinguish instance or device LayerRef

Merge Get*LayerRef into GetLayerRef and remove LayerRef::is_instance_.
With the removal, LayerRef::SupportsExtension becomes ambiguous.  Replace
it with FindLayer*Extension.  Remove unused LayerRef::GetName and
LayerRef::GetSpecName while at it.

There should be no user-visible change.

Bug: 27911856
Change-Id: I38340654b52338e9ed46d1c4462a6b254b0cab64